1. Field of the Invention
This invention relates to an optical aligner and, more particularly, to an optical sensor equipped with a luminance sensor.
2. Description of the Related Art
In a fabrication process of semiconductor integrated circuit device, various patterns are transferred onto a semiconductor wafer through lithographic techniques, and an optical reduction projection aligner usually transfers the patterns from masks to the semiconductor wafer. The optical reduction projection aligner presently available can transfer 0.5 micron line width for commercial products through an enlargement of numerical aperture of the projection optics. Although the enlargement of numerical aperture enhances the resolution, the depth of focus is decreased, and the trade-off between the resolution and the depth of focus does not allow the optical reduction projection aligner to transfer a line width equal to or less than 0.35 micron at an acceptable throughput for commercial products.
An attractive illumination system is disclosed in Japanese Patent Publication of Unexamined Application No. 61-91662, and FIG. 1 illustrates the prior art projection aligner disclosed in the Japanese Patent Publication of the Unexamined Application. The prior art projection aligner comprises a light source 1, elliptical mirrors 2a and 2b, a cold mirror 3 and a shutter 4. The light source 1 is implemented by a ultra high-pressure mercury lamp, and generates continuous spectrum light. The elliptical mirrors 2a and 2b condense and reflect the light toward the cold mirror 3, and the cold mirror 3 eliminates long-wavelength light from the light. The cold mirror 3 reflects ultra violet and visible light toward the shutter 4. In this instance, the g-line at 436 nanometer wavelength or the i-line at 365 nanometer wavelength is used in the pattern transfer, and these rays are majority in the reflected visible light. The shutter 4 selectively transmits and cuts off the reflected visible light.
The prior art projection aligner further comprises a lens 5 and an interference filter 6. The lens 5 makes the reflected visible light parallel, and the interference filter 6 only transmits the reflected visible or ultra violet light with the wavelength at 436 nanometers or 365 nanometers plus minus several nanometers.
The prior art projection aligner further comprises a fly-eye lens 8 implemented by elongated rectangular single lens units bundled together, and each single lens unit focuses for forming a light source group. The ultra high-pressure mercury lamp 1 radiates light which is not so coherent. The light passing through the fly-eye lens 7 makes a point source, and is enhanced in coherence. The point sources are called "effective light sources".
The prior art projection aligner further comprises a diaphragm 8 for shaping the light from the effective light source 7, a masking system 9, lens units 10 and 11 provided on both sides of the masking system 9, a mirror 12 for reflecting the light toward a reticle 13, a lens unit 14 provided between the mirror 12 and the reticle 13 and a projection lens system 15. The lens 10 focuses the light passing through the diaphragm 8 on the masking system 9, because the light focused on the masking system 9 restricts the diffraction due to the edge of the masking blade of the system 9.
The masking system 9 limits the exposure area, and the lens unit 11 and the mirror 12 change the optical path, and the change of the optical path makes the prior art projection aligner small. The reticle 13 is uniformly illuminated by virtue of the fly-eye lens 7, and the light transfers the pattern image through the projection lens system 15 to a photo-sensitive layer 16 coating a semiconductor wafer 17 mounted on an x-y stage 18. The photo-sensitive layer 16 is partially polymerized or depolymerized by the image-carrying light, and is, thereafter, developed for providing a mask layer on the semiconductor wafer 17.
Subsequently, description is made on an influence of the diaphragm 8 after the fly-eye lens 7 with reference to FIGS. 2A to 2D of the drawings. As shown in FIG. 2A, the diaphragm 8' usually has a circular opening 8a, and determines the numerical aperture NA of the projection optics. The numerical aperture NA of the projection optics affects the resolution characteristics, and sigma is defined as a ratio between NA of the illumination system and NA of the projection lens system 15. Sigma indicates the magnitude of the effective light source, and sigma is selected between 0.3 to 0.7 depending upon the image pattern of the reticle 13. For example, a line-and-space pattern formed in the reticle 13 requires relatively large sigma, and relatively small sigma is appropriate for an image pattern for a contact hole.
Moreover, the shape of the effective light source is optimized. For example, a diaphragm 8" may have a central shield area 8b so as to form a ring-shaped effective light source as shown in FIG. 2B. In general, 0-order light and either +1 or -1 order light are available for the resolution of the image pattern in the reticle 8. However, if the image pattern is miniaturized, the diffraction angle becomes large, and the light is hardly incident into the projection lens system 15. For this reason, the diaphragm 8' with the circular opening 8a allows only 0-order light L0 to pass through a central area thereof, and only the 0-order light L0 is incident into the projection lens system 15. The 0-order light L0 decreases the contrast, and the pattern image is hardly transferred to the photo-sensitive layer 16 due to the poor contrast.
However, if the diaphragm 8" with the ring-shaped opening 8c is used for the pattern transfer as shown in FIG. 2D, only an oblique light L1 is incident into the reticle 13, and either +1 or -1 order light passes through the projection lens system 15 to the photo sensitive layer 16. As a result, the miniature pattern is transferred to the photo-sensitive layer 16. Thus, the ring-shaped illumination effectively transfers a miniature pattern.
In order to improve the resolution by using the oblique light L1 as similar to the ring-shaped illumination, a diaphragm has four small openings, and illuminates a reticle through the four openings. However, the pattern transfer characteristics of the diaphragm is varied depending upon the shield area, and reticles with different image patterns require different diaphragms. Illumination systems using those diaphragms are called as an oblique illuminations or modified illuminations because of the light sources variable in shape.
However, the modified illumination system partially masks the parallel rays supplied from a fly-eye lens, and deteriorates the uniformity in illuminance. Moreover, the dispersion of the illuminance is varied with the shape of the opening in the diaphragm. For this reason, whenever an operator changes a diaphragm to another diaphragm different in shape of the opening, the operator measures the dispersion of the illuminance, and an adjusting work is required.
The measurement of the illuminance and the adjusting work are described hereinbelow. Turning to FIG. 1 of the drawings, photo-sensors 19 are put on the x-y stage. The operator moves the x-y stage 18 at short intervals in the x and y directions, and the shutter 4 are released in synchronism with the movement of the photo-sensors 19. The illuminance thus measured is recorded, and the x-y stage 18 causes the photo-sensors 19 to measure the illuminance over the exposure area. If the exposure area is not uniformly illuminated, the operator manipulates a regulating mechanism 19a for the ultra high-pressure mercury lamp 1, and regulates the position of the ultra high-pressure mercury lamp 1. The regulating mechanism 19a moves the ultra high-pressure mercury lamp 1 in the orthogonal coordinates x-y-z. The regulation of the ultra high-pressure mercury lamp 1 depends upon the experience of the operator, and, for this reason, the measurement of the illumination and the regulation are alternately carried out until the uniformity of illuminance satisfies a specification.
The trial-and-error regulation for uniform illumination is replaced with an automatic regulation. Japanese Patent Publication of Unexamined Application No. 63-70419 discloses the automatically regulable projection aligner, and FIG. 3 illustrates the automatically regulable projection aligner. Main component parts of the automatically regulable projection aligner are similar to those of the prior art projection aligner shown in FIG. 1, and are labeled with the same references. Differences are photo-sensors 20a, 20b, 20c and 20d provided on the masking system 9, a computer 21 and a driving system 22 for actuating the regulating mechanism 19.
As will be better seen in FIG. 4, the masking system 11a places the four photo-sensors 20a, 20b, 20c and 20d in a peripheral sub-area of the exposure area 23, and each of the photo-sensors 20a to 20d reports the illuminance to the computer 21. The computer 21 checks the reported illuminances to see whether to regulate the position of the ultra high-pressure mercury lamp 1 or not. If the regulation is necessary, the computer 21 instructs the driving system to actuate the regulating mechanism 19, and regulates the illuminances at the photo-sensors 20a to 20d to an acceptable range.
However, there is a trade-off in the prior art projection aligners between the accuracy of the regulation and the time consumed in the regulation. Namely, the operator manually moves the x-y stage over the exposure area for measuring the illuminance, and manually regulates the position of the ultra high-pressure mercury lamp. For this reason, the dispersion of illuminance over the exposure area exactly falls within an expected value defined in the specification. However, the trial-and-error regulation is time consuming. On the other hand, the prior art projection aligner shown in FIG. 3 quickly regulates the position of the ultra high-pressure mercury lamp 1 by virtue of the computer unit 21 communicating with the photo-sensors 20a to 20d. However, the photo-sensors 20a to 20d only monitor the fixed four points in the exposure area 23, and the accuracy is lower than that of the manually regulated projection aligner.
The pattern image of the reticle is progressively miniaturized, and the modified illumination or the oblique illumination using the non-circular opening is indispensable for transferring the miniature pattern image. For this reason, the semiconductor manufacturer needs a solution for the trade-off.